Blender 2.8 uses CPU rather than GPU, contradicting with settings
As @RobertGützkow already mentioned in the comment, you have not only selected your GPU for rendering, BUT also the CPU.

Selecting GPU Compute makes Blender render with all selected devices to speed up the entire procedure, while sometimes it is faster, it may even be slower, depending on the render and the devices as well as the render performance settings like bucket-sizes.

If you only select the GPU, as in un-check the CPU, Blender will render only on the GPU, but even then Blender needs to prepare some things beforehand and will use your CPU a little. After which the CPU should be left rather idle compared to what you experienced.

The Solution thus would be to make sure to not have the CPU selected in the render device settings.

Maybe it could be reasonable to change the GPU Compute to Compute on all selected Devices well a shorter name could be helpful, but i believe the you get the point.